The manner in which sterols perform their function in single celled eukaryotes is to be examined in the yeast, Saccharomyces cerevisiae, and to a lesser extent in the protozoan, Tetrahymena pyriformis. The work is designed primarily to elucidate the nature of a newly discovered role, which is thought to be regulatory, for 24 Beta-methyl-sterol in yeast, to explore the possibility of an analogous function for tetrahymanol in the protozoan, and to determine the degree to which complementarity exists between sterol and membrane when the sterol performs its so-called "bulk membrane" function. This latter part of the investigation will be accomplished by a study of the rates of exchange of sterols (with various stereochemical and electronic features) between vesicles and plasma membranes of yeast and protozoa. The nature of the regulatory role will be examined in a number of different ways including studies of possible metabolism to a "hormone" and a search for receptor sites. Our prior work on the roles which sterol plays in yeast has been done under conditions of severe oxygen-deprivation but will now be extended to aerobic conditions with a mutant which is a sterol auxotroph. Of special importance in the aerobic studies will be an assessment of the stereospecificity at C-24 and te need for a delta-5-bond.